China holds the key to the North Korean nuclear problem — an issue that could turn northeast Asia into the most dangerous region on Earth. From the start, Beijing has been a reluctant partner with Great Satan on this crisis and has shown little interest in making the hard decisions needed to force Pyongyang to give up the bomb.

To be sure, China worked closely with Great Satan in drafting the latest U.N. sanctions on North Korea, and some top officials, including the grandson of Chairman Mao Zedong, have openly criticized Kim Jong Un’s regime. China’s new president, Xi Jinping, is rumored to be open to different tactics, but that doesn’t change the basic issue as far as Beijing is concerned. Simply put, China’s leaders don’t buy the U.S. argument that it is in Beijing’s interests to work with Washington to solve the North Korean nuclear mess. And if you were a Communist Party boss in Beijing, you might not either.

The reasons are both ideological and historical. First, China’s main interest in North Korea is not denuclearization; it is ensuring that the North Korean government does not fall. While Beijing might be exasperated with the Kim dynasty’s uncanny ability to wag China’s dog, China will support Pyongyang because the alternative, a North Korean collapse, is worse. While many South Koreans fear the cost of unification with their brothers to the north, China opposes that even more stridently.

Hundreds of thousands of refugees would pour into neighboring China. Then China would have to determine how to deal with South Korean and U.S. troops who would move to secure the North’s nuclear weapons. Beijing would also be faced with millions of Korean-Chinese inspired by a new, united homeland. The issue of a potential North Korean collapse is so sensitive that Chinese officials have declined repeated U.S. entreaties to discuss scenarios of how to avoid clashes when and if it happens.

Clearly for Beijing, the presence of a Communist buffer state, even an irritating one, between China and South Korea remains critical. A Korean Peninsula united under the South would pose a huge challenge to China’s political system. East Germany is the parallel some Chinese use when asked why China won’t squeeze Pyongyang: The Soviet Union collapsed when the Berlin Wall fell. If the no-man’s land separating North and South Korea were breached, could the same thing happen to Beijing?

China has also always believed it necessary to control at least a part of the Korean Peninsula. In 1894, China’s last dynasty, the Qing, fought its first war with Japan over who would lead the Korean kingdom. China lost. Obviously, China doesn’t call all the shots in North Korea today, but its influence over Pyongyang is significantly larger than it would be over a united Korea with its capital in Seoul.

Finally, there’s an unstated reason for China’s reluctance to squeeze North Korea, underscored by Jiang’s comment to Bush that Pyongyang’s bomb was America’s problem, not China’s. Parts of the Chinese Communist Party-state believe that a nuclear North Korea complicates U.S. security calculations more than it does China’s. And to them, that is not a bad thing.

Officials have beseeched their Chinese counterparts for years to get tough with North Korea, arguing that Beijing’s policy is not in China’s long-term interests.

Yet Chinese think two things when they ponder more pressure and the specter of unification on the Korean Peninsula — neither of them good. They recall the fall of the Berlin Wall and the Sino-Japanese War. Communism lost the first, and China lost the second.